1. Field of the Invention
The present invention relates to position insensitive shock sensors, and more particularly to position insensitive shock sensors using conductive liquids to complete an electric circuit.
2. Description of Related Art
A variety of shock or deceleration/acceleration sensors have been developed in the past.
One type of shock sensor includes a weight, electrically connected to a terminal contact, suspended by a coil spring above a second terminal contact. When subjected to an appropriate shock, the weight overcomes the spring force and makes contact with the second contact, thus completing an electric circuit. A problem with these spring-type shock sensors is that vibration is created as a result of a shock. A related problem is that the electrical connection between the two contacts will be repeatedly interrupted because of the bounce of the spring supported contact. Such interruptions can promote contact degradation and produce erroneous signals. An additional problem is that the electrical circuit must usually include the spring supporting the weight. For sensors designed to be used in low G fields, the spring will have a fairly small diameter and/or a long wire length. These factors add undesirable electrical resistance to the circuit.
Another type of shock sensor includes a weighted contact supported on a flexible cantilever-type spring. These sensors also suffer from the vibration and the contact-bounce problems discussed above with respect to the coil-spring shock sensors.
Another type of shock sensor includes a strain gage mounted on a cantilevered plate that is designed to deflect in the region where the strain gage is mounted under shock or deceleration/acceleration forces. These sensors are relatively expensive to produce, and the electronics required to interpret the strain gage signals can be bulky.
Another type of shock sensor includes a flexible diaphragm spring that is suspended above a terminal contact. The diaphragm spring is connected to a second contact and is wetted with a thin layer of mercury on the surface facing the terminal contact. In the event of a shock, the diaphragm spring is deflected such that the mercury wetted surface contacts the terminal contact. Such a device does not have the above-described problems associated with contact bounce, but may not be usable over a wide range of G forces.
An additional problem with the above-described shock sensors is that they contain moving shaped-fixed, i.e., solid, parts that can be permanently deformed during a large shock.
Problems generally associated with spring-based shock sensors may be avoided by several other proposed devices which incorporate liquids, such as mercury, that are capable of conducting electricity. Such mercury based shock sensors would not generally need to include moving solid parts.
One such device is disclosed in U.S. Pat. No. 4,138,600. That patent discloses a force-responsive device that includes a housing having an hourglass-shaped interior with electric contacts projecting into it. The interior of the housing is not mercury-wettable. A contact medium, such as mercury, is governed by capillary adhesion, surface tension, and acceleration or deceleration forces to move within the housing interior to make or break electric circuits between the contacts. Actuation of the disclosed sensor causes mercury separation. The mercury must then be recollected by an external action.
Another mercury-based device is disclosed in U.S. Pat. No. 3,739,191. That patent discloses an acceleration/deceleration sensor having a tubular member with an electrode at one end and a so-called "deformable" element at the second end. Rapid acceleration/deceleration causes mercury to shift toward that second end of the tubular member and accumulate thereby creating a cavity at the other end of the member having the electrode. The interior of the sensor is not mercury-wettable. With the mercury separated from the electrode, the circuit is broken. The cavity is intended to create a vacuum for returning the mercury to its original position, and under a long-term deceleration/acceleration, the vacuum may be lost.
Another shock sensor is disclosed in U.S. Pat. No. 4,219,708. That patent discloses a shock switch that comprises a reservoir of conductive liquid held by surface tension against a non-mercury-wettable surface above a nonconductive region of gas. In the event of a shock or acceleration/deceleration, the surface tension is overcome causing the liquid to drop onto a pair of closely spaced electrical contacts and complete a circuit. The device is reset by an external force to return the conductive liquid to its position above the contacts.
Mercury-based operations have also been employed in relay switches intended to be operated by magnetic fields, not shocks. For example, attitude insensitive mercury relays are disclosed in U.S. Pat. Nos. 3,646,490; 3,697,906; 3,831,118; and 3,976,960. The disclosed relays include an enclosure having a flexible armature located within the enclosure adjacent an electric contact terminal. In each relay, the armature is mercury-wetted and covered with a thin layer of mercury that is intended to enhance the electrical contact between the armature and the contact. The mercury layer is not intended for protrusion from the armature, and is too thin for such protrusion. An electric coil adjacent the enclosure moves the armature into electrical contact with the terminal when the coil is energized by a current. In embodiments of U.S. Pat. No. 3,646,490, a space on the side of the armature opposite the contact terminal additionally is fillable by a layer of mercury (see, col. 3, lines 34-37).
A magnetic piston mercury switch is disclosed in U.S. Pat. No. 3,308,405. That patent discloses a switch having two mercury wettable tubes filled with mercury, the tubes being arranged in alignment with each other and separated by a mass of powdered iron. Contact terminals are arranged adjacent each opposite end. To activate the switch, a magnetic field draws the mass of iron toward one of the tube ends. This movement causes a displacement of the mercury within both tubes such that contact between the mercury and the contact terminals is either made or broken.
Mercury relays are also disclosed in U.S. Pat. Nos. 3,144,533; 3,715,546; 3,786,217; and 3,867,603. Those patents disclose relays having a shuttle slidably mounted within a tube. A thin layer of mercury is arranged between the shuttle and the tube to enhance electrical contacts and to reduce friction between the shuttle and the tube.